• From the Books Online:

    The missing indexes feature is on by default. No controls are provided to turn the feature on or off, or to reset any of the tables returned when the dynamic management objects are queried. When SQL Server is restarted, all of the missing index information is dropped.

    This feature can only be disabled if an instance of SQL Server is started by using the -x argument with the sqlservr command-prompt utility. For more information, see sqlservr Application.

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    Author of:
    SQL Server Execution Plans
    SQL Server Query Performance Tuning